#include <stdio.h>
#include <math.h>
#include <string.h>
#define N 200000
typedef struct 
{
    double x, y;
}POINT;

POINT polygon[N];
int n;    
double dd;
int in()
{
    int i;
    scanf("%d", &n);
    if (n<3) return 0;
    for (i=0; i<n; ++i)
    {
        scanf("%lf%lf", &polygon[i].x, &polygon[i].y);
    }    
    scanf("%lf", &dd);
    return 1;
}

double getarea()
{
    int i;
    double s;
    if (n<3) return 0;
    s = polygon[0].y*(polygon[n-1].x-polygon[1].x);
    for (i=1; i<n; ++i)
        s+=polygon[i].y*(polygon[i-1].x-polygon[(i+1)%n].x);
    
    return -s/2;
}    
    
void deal()
{
    double ans;
    ans = getarea();
    printf("BAR LENGTH: %.2lf\n", dd/ans);
}    

int main()
{
    while (in())
    {
        deal();
    }            
    return 0;    
}    



